Patent GB-495940-A (GB)
Title: Improvements in electric selective signal receiving arrangements
IPC: H04L15/26
Filed: 1938-03-23
Abstract: 495,940. Electric signalling systems. STANDARD TELEPHONES & CABLES, Ltd. March 23, 1938, No. 8943. Convention date, March 23, 1937. [Class 40 (i)] In a system for indicating the receipt of a predetermined signal code comprising a series of marks and spaces of predetermined tolerance, a counting device, e.g. a stepping switch or a counting relay chain is operated fully by a minimum mark timer in response to a correct signal, but is released under the control of a maximum mark timer or a space timer if the tolerances are exceeded. The space timer also releases the counting device if a subsequent mark is less than the predetermined duration. The Figure shows a system wherein an alarm 4 is controlled by a counting device 3 which s operated under the control of minimum and maximum mark timers 2, 5 and a space timer 6 which are responsive to the operation of a signal receiver 1. The system is designed to respond to marks of 3¢ to 4¢ seconds duration and spaces of less than 1¢ seconds, the arrangement shown being responsive to a signal including three marks. Correct operation. When the first mark is received by relay 100, the timer 230, which is timed to close its contacts 1, 2 after 3¢ seconds under the action of mechanical, electric or magnetic delay means, is started and provided the mark persists for 3¢ seconds relay 231 is pulled up to initiate the operation of maximum timer 530 which measures a further 1 second. In a modification, the maximum timer may be operated directly by relay 100 and arranged to measure 4¢ seconds. Upon the cessation of the first mark, relays 231, 100 release but owing to the slow release of relay 231, the magnet 333 is operated to step the counter 330 one step. The slowrelease of relay 231 also allows relay 230 to restore and prevents further stepping in response to a rapid succession of impulses. When the counter steps, the operation of the space timer 630 is initiated over the contact 1 of the counter 330 and back contact 2 of the timer 230. The timer 630 closes its contacts after 1¢ seconds and has no effect provided the relay 100 is operated by the second mark prior to the expiration of that period. Short dots or extraneous signals within the correct space period also are without effect. Upon receipt of a correct second mark, timers 230, 530 are again operated, the counter is further operated and the timer 630 is released at the back contact 2 of the timer 230. Similar operations occur in response to the third mark and the counter 330 operates relays 336 which operates the alarm 4 and prevents further operation of the timers 230, 530. The operation of the alarm may be nullified and the complete system restored to normal by a key 337 which when depressed operates the release magnet 335 of the counter. Incorrect operation. If a mark is longer than 4¢ seconds, the timer 530 operates the release magnet 335 to wipe out previous counts. Restoration of the counter 330 also is effected by timer 630 over the back contacts of relay 100 if the space is too long or in the event of the release of the relay after a short pulse at the termination of the space period or a break in the second mark. Modifications. In a modified system, the timers comprise thermionic valves the operations of which are timed by resistance-capacity networks connected in the grid circuits ; in this system the marks are counted by a relay chain consisting of pairs of relays operated on the two-step principle by the application and cessation of the marks. The system operates in a manner similar to that previously described but is also adapted to restore to normal upon failure of any one of the valve filaments. In a further modification, timing is effected by cams adapted to be clutched to the shaft of a constant speed motor. In this arrangement, the speed of the motor is automatically regulated by a short-circuiting resistor in the armature circuit and the alarm is operated if the speed varies beyond a predetermined value.